1. Field of the Invention
The present invention relates to a non-volatile memory device, specifically, an EEPROM, and particularly to a method of controlling a reference voltage which refers to stored information upon its reading.
2. Description of the Related Art
As shown in FIG. 6, the non-volatile memory device has heretofore been provided with current comparing cells disposed in one respective cell blocks in one rows and circuits each of which compares a current flowing through one comparing cell and a current flowing through one cell that reads stored information to thereby determine 0 or 1. Details of an AMP's circuit shown in FIG. 6 are given as illustrated in FIG. 7. An AMP10 includes an AMP1, an AMP2 and an AMP3. The AMP1 is provided with a reading detector and a comparing detector. The comparing detector is twice as large in dimension as the reading detector. The current always flows in the corresponding comparing cell connected to the comparing detector without information being written therein.
When 1 indicative of stored information is read, the same current is caused to flow through a cell for reading the stored information and a comparing cell, and a current flowing from a comparing BL is always added to a current that flows through the comparing detector. Therefore, an AMP1 output outputted from the reading detector results in a value higher than VREF due to the ratio between the dimensions of the reading detector and the comparing detector. When 0 indicative of stored information is read in reverse, no current flows through the corresponding cell for reading the stored information, and hence the AMP1 output becomes lower than VREF. Incidentally, VREF becomes an intermediate potential of the AMP1 output where 1 and 0 of the stored information are read. The contents referred to above have been described in, for example a patent document 1 (Japanese Unexamined Patent Publication No. Hei 09(1997)-097496).
However, the AMP10 might cause a malfunction. Even though a reading cell is found not to be defective, it might be detected as defective. The following two examples are considered as malfunction's mechanisms.
The first example shows the case in which a comparing cell is reduced in current at its turning-on due to its process variations and an information reading cell increases in off leak current due to its process variations. As a result of the reduction in the current flowing through the comparing cell, the value of VREF become low. As a result of the increase in the current flowing through the reading cell, the AMP1 output becomes high and hence the AMP1 output exceeds the value of VREF, so that the AMP10 is considered to lead to the malfunction.
The second example shows the case in which a comparing cell increases in current at its turning-on due to its process variations and an information reading cell is reduced in on current due to its process variations. As a result of the increase in the current flowing through the comparing cell, the value of VREF becomes large. As a result of the reduction in the current flowing through the reading cell, the AMP1 output becomes small and hence the AMP1 output exceeds the value of VREF, so that the AMP10 is considered to lead to the malfunction.
The two cases arise together where the process variations related to the comparing and reading cells are noticeable. If the process variations related to either the comparing cell or the reading cell can be less reduced, then the present problem can be solved.